1. Field of the Invention
This invention relates to a sintered body having a high alumina content, and more specifically, to a high alumina sintered body having excellent thermal shock resistance and flexural strength.
2. Description of the Prior Art
High alumina ceramics containing at least 75% of Al.sub.2 O.sub.3 (to be referred to as high alumina ceramics) have heretofore found wide applications in electronic component parts, industrial machine parts, etc. because of their excellent electrical insulation, heat resistance and mechanical properties.
It is known however that high alumina ceramics of this type have relatively high coefficients of thermal expansion among ceramics which are at least 70.times.10.sup.-7 /.degree. C., and cracks occur when the thermal shock temperature difference in rapid cooling in water is 200.degree. C., and thus they have poor thermal shock resistance characteristics.
To improve thermal shock resistance, there have been used means for inhibiting the occurrence of thermal shock cracks by using materials having a low coefficient of thermal expansion such as aluminum titanate and cordierite, or rendering the sintered body porous. In such cases, however, the sintered bodies have a very low mechanical strength represented by a flexural strength of not more than 15 kg/mm.sup.2.
Recently, Japanese Laid-Open Patent Publication No. 73665/1981 disclosed a low expansion high strength ceramic composition composed of mullite particles, alumina particles and a matrix of a B.sub.2 O.sub.3 -containing glass phase filling the interstices among these particles. Although this ceramic composition has a low coefficient of linear expansion as compared with conventional high alumina ceramics, it has a flexural strength of as low as 30 kg/mm.sup.2, and its mechanical strength is still unsatisfactory.